Analysis and Spectra Flashcards
Molecular spectra, whether NMR, IR, or mass spec, can be intimidating when they appear on the MCAT. Use these cards to master equivalent protons, memorize important shifts, and learn how these complex methods relate.
What do infrared (IR) and ultraviolet (UV) spectroscopy have in common?
Both techniques involve absorption of electromagnetic radiation by the molecule that is being analyzed.
Radiation with a specific frequency will be absorbed by the electrons in a molecule, exciting them to higher energy states.
What happens if a molecule is simultaneously exposed to several frequencies of light?
The molecule will only absorb radiation from the particular frequencies at which it has matching energy states. This concept is known as resonance.
Radiation with other frequencies will pass through or reflect. Spectroscopy, or the process of measuring the absorbed radiation, can yield information about the composition of the molecule.
What can infrared (IR) spectroscopy identify about an organic molecule?
functional groups present on the molecule
Different covalent bonds vibrate at different, characteristic frequencies when light is absorbed by the molecule. These frequencies correspond to certain common groups.
In infrared (IR) spectroscopy, how can molecules with exactly the same atoms absorb light differently?
Different covalent bonds vibrate at different, characteristic frequencies. These vibrations can exist in different forms, such as stretching, bending, and rocking.
Molecules that contain the same atoms but different functional groups will absorb light at their specific characteristic frequencies. Their spectra will not necessarily overlap.
What determines the frequency of an IR vibration?
- number of electron pairs
- polarity of the bond
- masses of the atoms involved
What qualities characterize bonds with high IR vibration frequencies?
- double and triple bonds
- polar bonds
- bonds that include lighter atoms
Remember, IR vibration frequencies are determined by the characteristics of the bond.
What qualities characterize bonds with low IR vibration frequencies?
- single bonds
- nonpolar bonds
- bonds that include heavier atoms
Remember, IR vibration frequencies are determined by the characteristics of the bond.
How can a molecule be identified using an IR spectrum?
It is identified via characteristic peaks in its spectrum.
Though functional groups may have many absorption peaks across the full spectrum, certain ranges are important to know.
Which functional group yields the IR peak shown below?
hydroxyl (-OH)
Hydroxyl groups always give a broad absorption peak in the range of 3100-3500 cm-1.
Which functional group yields the IR peak shown below?
carbonyl (C=O)
Carbonyl groups always give a sharp absorption peak around 1700 cm-1.
Which functional group yields the IR peak shown below?
amine (N-H)
Amine groups always give a sharp absorption peak in the range of 3100-3500 cm-1.
What functional group must be present in a molecule with the IR spectrum shown below?
carbonyl
Carbonyl groups always give a sharp absorption peak around 1700 cm-1. The graph above is the spectrum of propanone.
What functional group must be present in a molecule with the IR spectrum shown below?
hydroxyl
Hydroxyl groups always give a broad absorption peak in the range of 3100-3500 cm-1. The graph above is the spectrum of propan-1-ol.
What functional group must be present in a molecule with the IR spectrum shown below?
amine
Amine groups always give a sharp absorption peak in the range of 3100-3500 cm-1. The graph above is the spectrum of 1-aminobutane.
What functional group must be present in a molecule with the IR spectrum shown below?
hydroxyl
Hydroxyl groups always give a broad absorption peak in the range of 3100-3500 cm-1. The graph above is the spectrum of ethanol.
What is the fingerprint region of an IR spectrum?
The area of the spectrum between 500 and 1450 cm-1.
Every molecule’s fingerprint region is unique, but the signal in this area tends to be complex. For the MCAT, don’t worry about identifying peaks in this area.
What region of the IR spectroscopy spectrum can generally be disregarded?
The fingerprint region, which covers the range from 500 cm-1 to 1450 cm-1.
The fingerprint region often does not contain distinct characteristic peaks.
What can UV-visible spectroscopy identify about an organic molecule?
electronic energy levels
Electrons bound to different molecules will have different excited states. These molecules will absorb photons of light that correspond to the difference between the ground and excited energy levels, which exist in the visible and near-UV regions of the spectrum.
What features characterize the organic molecules that display signals in UV-visible spectroscopy?
It must have nonbonding electrons that can be excited without breaking a bond.
The most common examples are:
- conjugated molecules (most important for the MCAT)
- molecules with double and triple bonds
- molecules with nonbonding pi electrons
- molecules that contain transition metals
What does the color of an organic molecule indicate about the frequency of light that it absorbs?
Molecules are complementary in color to the light that they absorb.
For example, if a molecule is illuminated with white light and absorbs green light, it must reflect light from the remainder of the spectrum, namely red. The molecule will appear reddish in color.
What color of light must be absorbed by a molecule that appears reddish in color?
green light
More broadly, the absorbed light must have a frequency around the yellow/green/blue area of the spectrum.
A molecule that absorbs green light will reflect red, making it appear reddish in color.
What color of light must be absorbed by a molecule that appears greenish in color?
red light
A molecule that absorbs red light will reflect light in the blue/green/yellow area of the spectrum, causing it to appear green in color.
What does a mass spectrometer measure?
It measures the path curvature of molecular fragments from a sample. This is then related to a “mass-to-charge ratio” for each fragment.
The mass-to-charge ratio can then be used to calculate molecular mass.
Briefly describe how a mass spectrometer works.
It bombards a vapor sample with sufficiently high-energy electrons to both fragment and ionize the sample.
The charged ions and fragments are then accelerated using an electric field and bent through a curved detector using a magnetic field. The radius of curvature for the ions depends on their mass and charge, allowing total mass to be calculated.
A student is looking for a laboratory technique that will help him separate two components of a sample. What conditions must be satisfied for mass spectrometry to be a good choice?
Mass spectrometry is never a good separation technique. It is used for analysis, not separation / purification.
On the MCAT, be careful to distinguish separation techniques from analytic ones. Mass spectrometry, for example, destroys the molecule that is being analyzed, so it would not be used to separate components of a mixture for later use. Actual separation techniques include extraction and distillation.
What does the “NMR” stand for in NMR spectroscopy?
Nuclear Magnetic Resonance
Although there are multiple types of this process, only proton (H+) NMR is tested explicitly on the MCAT.
NMR operates by exposing a molecule in the solvent phase to a large, permanent magnetic field. It then measures its resonant absorptions of electromagnetic radiation.
At the molecular level, what does H+ NMR spectroscopy measure?
It measures the environment of the protons, or hydrogen atoms, in a molecule.
Hydrogens bound to different atoms, and in different configurations, will respond differently to magnetic fields. This concept forms the essence of H+ NMR spectroscopy.
What determines the chemical shift of a proton in NMR spectroscopy?
It depends on how deshielded it is by the other atoms in the molecule.
Deshielding generally refers to the proton’s proximity to electronegative groups. The less electron density an atom retains, the more deshielded it is, and the further downfield its chemical shift will be. Resonance structures and highly electronegative atoms tend to deshield protons.
A proton near a certain functional group displays an NMR peak of 9.5 ppm. Would this peak be considered to be upfield or downfield?
It would be considered to be downfield. NMR peaks are relative, but the more downfield a peak is shifted, the higher its value will be. 9.5 ppm is significantly higher than a characteristic alkane peak around 0-4 ppm.
Downfield shifts are the result of deshielding, usually due to proximity to electronegative groups. 9.5 ppm is near the characteristic shift for an aldehyde.
A certain NMR spectrum displays multiple distinct peaks. What does this show about the molecule?
It contains non-identical protons. Signals in an NMR spectrum represent different chemical shifts due to different proton environments.
Identical protons will display signals at the same point, such as the protons in methane. In contrast, a molecule with many different proton environments will have multiple peaks, such as butanoic acid (4 peaks).
A certain NMR signal has an integrated area three times that of another peak. What does this show about the protons involved?
The set of equivalent protons corresponding to the larger peak must contain three times as many protons as the other. The area of an NMR peak is proportional to the number of identical protons with the given chemical shift.
Identical protons will have signals at the same point, and their signals will add up to a single peak in the measured spectrum. On the MCAT, integrated area is often given if it is necessary, but peak height can also be used as a valid approximation.
In what range is the NMR chemical shift of a proton bound to an alkane (sp3-hybridized)?
between 0 and 4 ppm
For example, the protons on ethane have an NMR value of about 0.86.
In what range is the NMR chemical shift of a proton bound to an alkene (sp2-hybridized) carbon?
between 4 and 7 ppm
For example, the protons on ethene have an NMR value of about 5.35.
In what range is the NMR chemical shift of a proton bound to an aromatic carbon?
between 6 and 8 ppm
For example, the protons on benzene have an NMR value of about 7.28.
By what amount does an adjacent electronegative atom usually affect the NMR chemical shift of a proton?
a downfield shift of up to 3 ppm
For example, a proton in methane has a peak at about 0.25 ppm, while the chemical shift of a proton in chloromethane is around 3.0 ppm. The electronegative chlorine atom draws electron density towards itself, deshielding the other protons in the molecule by 2.75 ppm.
In what range is the NMR chemical shift of a proton bound to an aldehydic (CHO) carbon?
a downfield chemical shift between 8.5 and 11 ppm.
For the MCAT, simply remember this value as around 9.5 ppm.
For example, the protons on formaldehyde have an NMR value of about 9.61.
In what range is the NMR chemical shift of a carboxylic acid (COOH) proton?
a downfield chemical shift between 10 and 12.5 ppm
For example, the carboxylic acid proton on acetic acid has an NMR value of about 11.8.
What types of protons are likely present in the molecule whose NMR spectrum appears below?
This molecule has only one peak in the range for aromatic protons, so all protons are likely part of an aromatic structure.
This is the NMR spectrum of benzene, a common organic molecule often mentioned on the MCAT.
What types of protons are likely present in the molecule whose NMR spectrum appears below?
This molecule has two unique peaks in the sp3-hybridized range, probably corresponding to alkane protons. It also contains a single proton in the aldehydic range.
This is the NMR spectrum of propanal, an aldehyde.
What does spin-spin splitting look like in an NMR spectrum?
It results in peaks that look jagged. A peak that displays splitting will look like multiple peaks packed extremely close together.
However, such a peak will still only refer to one proton or set of equivalent protons.
In proton NMR, how can one determine the amount of splitting of a single peak?
Each unique proton’s peak is split into (n+1) subpeaks, where n is the number of protons bound to adjacent carbons.
“Adjacent” refers to carbons next to the carbon to which the proton in question is bound.
With regard to splitting, what name is given to the blue peak (far right) in the NMR spectrum below, and how many adjacent protons does it represent?
It is a doublet, meaning that the adjacent carbon is bound to one proton.
Since the peak is split into a doublet (2 subpeaks), the (n+1) rule states that a single adjacent proton must be present (n = 1). To recall the n+1 rule, remember that a peak corresponding to a proton with zero adjacent protons will be a singlet.
The spectrum shown above is of propene.
